The present invention relates to a compound sheet gasket which is mounted, for example, between a transmission housing and a cover, for blocking an opening at a brim of the housing, and in particular, relates to a technology for maintaining superior sealing properties which prevent the compound sheet gasket from being damaged by fretting.
Generally, heat resistance is not required in a gasket used in a transmission housing for automobiles, or the like, since the internal oil temperature is about 140 to 150° C. Therefore, as the gasket, an inexpensive sheet gasket consisting of nonmetal compound material sheets can be used, and a metallic gasket having a bead such as a gasket for an internal combustion engine cannot be used. The sheet gasket is mounted between flanges formed at brims of a housing and a cover and is adhered to the flanges due to the flexibility thereof, and it thereby prevents transmission oil from leaking.
FIGS. 5A, 5B, 6A, and 6B are drawings showing states in which a conventional sheet gasket is mounted in a transmission housing. In the figures, numeral 1 is a flange of the transmission housing and numeral 2 is a flange of the cover. A sheet gasket 3 is mounted between these flanges 1 and 2 and is tightly fastened by bolts 4.
Fretting occurs between the flanges 1 and 2 due to external stresses, thermal expansion, vibrations, or the like, which affect the transmission housing. As shown by the double-headed arrows in the figures, sliding occurs on the front surface and the rear surface of the sheet gasket by this fretting, and during this time, the sheet gasket is often shifted from an original position by friction resistance (shown by two arrows in the figure) which occurred due to the sliding, as shown in FIG. 6A. Consequently, there was a problem in that sealing was remarkably lowered by the occurrence of deformations or cracks in the sheet gasket.
Therefore, the following countermeasures were considered in order to overcome the above problem.    {circle around (1)} The fastening axial force is increased by increasing the number of fastening bolts as well as increasing the rigidity of the flanges, and the fastening bearing is increased and the fretting amount is reduced.    {circle around (2)} The proof stress for fretting is improved by increasing the mechanical strength of the sheet gasket.    {circle around (3)} An interfitting structure such as a knocking pin, etc., is provided on a butting portion of the flanges, and the fretting in the flange plane is reduced.    {circle around (4)} The frictional force which occurred due to fretting is reduced on the surface of the sheet gasket by coating a solid lubricant, such as graphite, molybdenum disulfide, etc., and the stress which affects the sheet gasket is reduced.
However, the above countermeasures have the following problems, respectively.    {circle around (1)} Addition of Fastening Bolts and Improvement in Rigidity of Flange
The manufacturing cost is increased by complicating the fastening structure of the flanges, and moreover, the weight is increased by the increase in number of parts and increase in size of the parts.    {circle around (2)} Improvement in Strength of Sheet Gasket
The compressibility is reduced by increasing the strength of the sheet gasket, and thereby the sealing property is deteriorated and the weight is increased.    {circle around (3)} Interfitting Mechanism
The manufacturing cost is increased by complicating the flange structure and by increasing the number of parts.    {circle around (4)} Coating of Solid Lubricant
By excessively increasing the sliding between the sheet gasket and the flange, the fretting amount is increased and deformation is promoted if the strength of the sheet gasket is low.
In addition, as disclosed in Japanese Utility Models Unexamined Publications Nos. 2-59362 and 3-124075, technologies constituted so that a metal gasket may slip off either of a cylinder head or cylinder block due to differences between friction resistances of the front surface and the rear surface of the metal gasket, are proposed in the technological field of metal gaskets. However, in either proposed technology, the difference in the friction resistance is not quantitatively analyzed, and it is not clear whether the above problem can be overcome by applying such technology to a sheet gasket.
Therefore, it is an object of the present invention to provide a compound sheet gasket which exhibits superior sealing durability to fretting.